In the semiconductor manufacturing industry, photoresist materials are used for transferring an image to one or more underlying layers, such as metal, semiconductor and dielectric layers, disposed on a semiconductor substrate, as well as to the substrate itself. To increase the integration density of semiconductor devices and allow for the formation of structures having dimensions in the nanometer range, photoresists and photolithography processing tools having high-resolution capabilities have been and continue to be developed.
Positive-tone chemically amplified photoresists are conventionally used for high-resolution processing. Such resists typically employ a resin having acid-labile leaving groups and a photoacid generator. Exposure to actinic radiation causes the acid generator to form an acid which, during post-exposure baking, causes cleavage of the acid-labile groups in the resin. This creates a difference in solubility characteristics between exposed and unexposed regions of the resist in an aqueous alkaline developer solution. Exposed regions of the resist are soluble in the aqueous alkaline developer and are removed from the substrate surface, whereas unexposed regions, which are insoluble in the developer, remain after development to form a positive image.
One approach to achieving nm-scale feature sizes in semiconductor devices is the use of short wavelengths of light, for example, 193 nm or less, during exposure of chemically amplified photoresists. To further improve lithographic performance, immersion lithography tools have been developed to effectively increase the numerical aperture (NA) of the lens of the imaging device, for example, a scanner having a KrF or ArF light source. This is accomplished by use of a relatively high refractive index fluid (i.e., an immersion fluid) between the last surface of the imaging device and the upper surface of the semiconductor wafer. The immersion fluid allows a greater amount of light to be focused into the resist layer than would occur with an air or inert gas medium. When using water as the immersion fluid, the maximum numerical aperture can be increased, for example, from 1.2 to 1.35. With such an increase in numerical aperture, it is possible to achieve a 40 nm half-pitch resolution in a single exposure process, thus allowing for improved design shrink. This standard immersion lithography process, however, is generally not suitable for manufacture of devices requiring greater resolution, for example, for the 32 nm and 22 nm half-pitch nodes.
Considerable effort has been made to extend the practical resolution capabilities of positive tone development in immersion lithography from both a materials and processing standpoint. One such example involves negative tone development (NTD) of a traditionally positive-type chemically amplified photoresist. NTD allows for use of the superior imaging quality obtained with bright field masks for printing the critical dark field layers. NTD resists typically employ a resin having acid-labile (or acid-cleavable) groups and a photoacid generator. Exposure to actinic radiation causes the photoacid generator to form an acid which, during post-exposure baking, causes cleavage of the acid-labile groups giving rise to a polarity switch in the exposed regions. As a result, a difference in solubility characteristics is created between exposed and unexposed regions of the resist such that unexposed regions of the resist can be removed by particular developers, typically organic developers such as ketones, esters or ethers, leaving behind a pattern created by the insoluble exposed regions. Such a process is described, for example, in U.S. Pat. No. 6,790,579, to Goodall et al. That document discloses a photoresist composition comprising an acid-generating initiator and a polycyclic polymer containing recurring acid labile pendant groups along the polymer backbone. The exposed areas can be selectively removed with an alkaline developer or, alternatively, the unexposed regions can be selectively removed by treatment with a suitable nonpolar solvent for negative tone development.
Conventional 193 nm photoresist polymers typically include alkyl adamantyl methacrylate units in which the alkyl adamantly moieties function as acid labile leaving groups. Such leaving groups, however, exhibit very slow dissolution rates in NTD developers such as 2-heptanone and n-butyl acetate (NBA). Low dissolution rates have been found to result in poor pattern fidelity. Increasing the dissolution rate of such polymers in an NTD developer can be accomplished by use of relatively low molecular weight polymers. This is not a viable total solution as lower molecular weight polymers have been found to exhibit poor photospeed and CD uniformity.
There is a continuing need in the art for improved photoresist compositions and photolithographic methods for negative tone development which allow for the formation of fine patterns in electronic device fabrication and which avoid or conspicuously ameliorate one or more of the foregoing problems associated with the state of the art.